The substrates of optical disks such as video disks, compact disks and disk memories are conventionally made of hard plastics such as methyl methacrylate and polycarbonates. Such an optical disk employes two substrates each having a recording surface and a metallic reflective layer formed in superposition on one side. The two substrates are positioned in such a manner that the reflective layer on one substrate will face the reflective layer on the other substrate and are bonded together, with a spacer being interposed therebetween over the entire surface except the center hole or in two selected areas, one being around the center hole and the other on the periphery.
Adhesives which are conventionally used in bonding optical disk substrates include solvent type adhesive (e.g. epoxy based adhesives), heat activatable adhesives, and two-component condensation type cold curable silicone rubber based adhesives. Solvent type adhesives have the disadvantage that they erode both the substrates themselves and the reflective films. Besides, they are unsuitable for mass production since the substrates have to be held stationary under uniform pressure applied for a long period of time until the adhesives are completely cured. Heat activatable adhesives have the advantage that they can be solidified in a short period of time after activation. On the other hand, the heat applied for activation gives thermal strain to the substrates to induce deformation of the substrates and the disk is unable to rotate uniformly to reproduce exact information.
In order to ensure that storage of an optical disk at a high temperature for a long period of time will not cause any deterioration in quality thereof, optical disks are required to have a shear adhesive strength of at least 10 kg/cm.sup.2 and a peeling strength of at least 0.7 kg/20 mm after they have been subjected to a wet heat test under the conditions of 70.degree. C..times.85% R.H. (relative humidity) for 2,000 hours. However, none of the optical disks has been yet developed to fully satisfy those requirements.